[tex-k] Reproducible builds using pdftex

Hello.
Working on the “reproducible builds” effort [1], we have noticed that a
lot of software packages rely on pdftex to build some documents
to be included in the binary package. Since revision 728 on pdftex's
svn, pdftex honours the SOURCE_DATE_EPOCH environment variable to get
reproducible values for the CreationDate, ModDate and ID fields in the
produced file. This greatly helps reproducibility.
However, a lot of software package date their documentation using the
`\today' command. This breaks reproducibility, and the document date
becomes the build date instead of the source files date as it should be.
Therefore, I would like to promote a feature for pdftex, that would use
(when set) the value of SOURCE_DATE_EPOCH to also feed the values of the
\year, \month and \day primitives.
Please find attached a patch that implements this feature, factorizing
the already present code from initstarttime to get and check the
SOURCE_DATE_EPOCH value, so that it can also be used in get_date_and_time.
Thanks in advance for considering including this feature in pdftex.
Regards,
Alexis Bienvenüe.
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